The present invention relates to an apparatus and a method for printing and applying solder paste onto surfaces of circuit boards.
In the prior art of manufacturing electronic circuit boards, solder paste has been primarily used to solder electronic components such as chips or the like onto printed boards. A solder paste printing apparatus has been employed to print and apply the solder paste in a required pattern.
A squeegee head loaded onto a conventional solder paste printing apparatus 100 is shown in FIG. 20. Generally, a squeegee head 102 moves from left to right and from right to left in FIG. 20, alternately for each printed board 5, to print the solder paste. A right squeegee 101a is used for printing in the right direction, namely, from left to right in FIG. 20, and a left squeegee 101b is used for printing in the opposite left direction.
Operation of the conventional solder paste printing apparatus 100 will be described with reference to FIGS. 20-22. In FIGS. 20-22, reference numbers respectively indicate: 3 a mask having openings 4 formed in a required pattern; 5 a printed board; 6 a land to which a solder paste 7 is printed; and 8 a solder resist. The above-mentioned required pattern of the mask 3 is a pattern in which the openings 4 are formed corresponding to the lands 6 on the printed board 5.
Firstly, during printing in the right direction, the printed board 5 is positioned and overlapped with the mask 3 so that the openings 4 align with the lands 6. Thereafter, while the left squeegee 101b is kept raised, the right squeegee 101a is lowered to bring a squeegee front end part 103 in touch (contact) with a surface 3a of the mask 3 with a suitable printing pressure. In this state, the right squeegee 101a is linearly moved in the right direction to thereby fill the solder paste 7 provided at the surface 3a of the mask 3 into the openings 4 of the mask 3. The printed board 5 is separated from the mask 3 after the right squeegee 110a is moved to the right end of the mask 3, when the printing is finished. Meanwhile, in the case of printing in the left direction, the printed board 5 is positioned and overlapped with the mask 3, which is similar to printing in the right direction. Then, the left squeegee 101b is lowered, while the right squeegee 110a is held up, to bring the squeegee front end part 103 in touch with the mask 3. The operation afterwards is the same as in the case of printing in the right direction. By repeating the operations in the right direction and left direction alternately for every printed board 5, the solder paste 7 is continuously printed and applied onto the lands 6 of every printed board 5 via the mask 3.
In the conventional solder paste printing apparatus 100, the squeegee 101a or 101b is moved while the front end part 103 of the squeegee is kept in touch with the surface 3a of the mask 3 with a suitable printing pressure. Each of the conventional squeegees 101a, 101b executes two kinds of operations (i.e., a scraping operation of scraping the solder paste 7 at the surface 3a of the mask 3 and a filling operation of filling the solder paste 7 into the openings 4 of the mask 3). These operations will be described with reference to FIGS. 21 and 22.
FIGS. 21 and 22 are enlarged views of the printing squeegee 101a, etc. when the printing is carried out in the right direction. As shown in FIG. 21, when the right squeegee 101a is lowered to bring the front end part 103 in touch with the surface 3a of the mask 3 and moved linearly in the right direction, the right squeegee 101a, which touches the solder paste 7 fed onto the surface 3a of the mask 3, moves while scraping the solder paste 7. The scraped solder paste 7 flows in a rotary direction called rolling as indicated by an arrow I in FIG. 22. At this time, a fluid pressure is generated inside the solder paste 7. When the right squeegee 101a moves further right to reach the opening 4 of the mask 3 in the above condition, the solder paste 7 is pressed into the openings 4 because of the above-described fluid pressure. In other words, the solder paste 7 is filled in the openings 4. A pressure with which the solder paste 7 is pressed into the openings 4 will be denoted as a xe2x80x9cfilling pressurexe2x80x9d hereinbelow.
A coordinate system is shown in FIG. 23 with the printing squeegee 101a. A viscosity of the solder paste 7 is xcex7. An angle of the surface 3a of the mask 104 of the printing squeegee 101a facing the surface 3a (referred to as a xe2x80x9csqueegee anglexe2x80x9d hereinafter) is xcex1. A speed of the moving squeegee 101a (referred to as a xe2x80x9csqueegee speedxe2x80x9d) is v. It is known that a fluid pressure p produced inside the solder paste 7 is expressed by an equation below:
p=(2xcex7v/r)*(A sinxcex8+B cosxcex8)xe2x80x83xe2x80x83(1)
wherein r is a radial position in a polar coordinate system of FIG. 23, xcex8 is an angle of the surface 3a of the mask 3 to the above r, A=sin2xcex1/(xcex12xe2x88x92sin2xcex1) and B=(xcex1xe2x88x92sinxcex1*cosxcex1)/(xcex12xe2x88x92sin2xcex1).
From the above expression (1), a fluid pressure distribution in the solder paste 7 and a pressure distribution at the face 104 of the printing squeegee 101a are as indicated in FIG. 24. Specifically, a shaded part 105 in FIG. 24 is where the fluid pressure p (adequate filling pressure) is generated.
Print time in the printing process is reduced in order to improve productivity. However, if the squeegee speed v is made faster in order to shorten the print time in the conventional solder paste printing apparatus 100, the amount of the solder paste 7 filled in the openings 4 of the mask 3 becomes less as indicated in FIG. 25. This results in printing failures such as non-filled parts 9 (unfilled parts) and hinders stable printing. The non-filled parts 9 are not generally brought about, for example, if ink is used for screen printing, but are created when a paste-like substance of a mixture of a high viscosity flux with powder solder (e.g., the solder paste 7 or the like is used). The non-filled parts 9 are caused by using the powder solder. In other words, it is impossible to realize a reduction of the print time through an increase of the squeegee speed v in the conventional solder paste printing apparatus 100.
How the non-filled part 9 results from an increase of the squeegee speed v is described as follows. When the squeegee speed v is increased (more than in the prior art), a time for the front end part 103 of the printing squeegee 101a to pass over the opening 4 is reduced. Consequently, a time for the solder paste 7 to be filled in the opening 4 (referred to as a xe2x80x9cfill timexe2x80x9d) is naturally shortened. The filling pressure becomes maximum when r=0 (i.e., at a contact point between the front end part 103 of the printing squeegee 101a and the surface 3a of the mask 3), as is understood from the expression (1) and FIGS. 23 and 24. Although pr=pxcex8=∞ is theoretically held when-r=0, the contact point is practically a stagnation point and accordingly is at a maximum value.
While the filling pressure itself is raised when the squeegee speed v is increased, a high pressure range is narrow as is seen from the pressure distribution of the shaded part 105 of FIG. 24. In addition, since the front end part 103 passes over the opening 4 instantaneously, the fill time cannot be long enough. As a result of this, the non-filled part 9 is formed.
In order to prevent an occurrence of the non-filled part 9, from the expression (1), it can be considered that the squeegee angle xcex1 should be reduced and at the same time the filling pressure should be increased, whereby the filling is completed even with a short fill time. However, since the conventional printing squeegee 101a performs two operations, namely, the filling operation of the solder paste 7 and the scraping operation from the surface 3a of the mask 3 as described above, the front end part 103 is largely deformed if the filling pressure is increased, which makes it impossible to scrape the solder paste 7 from the surface 3a of the mask 3. The solder paste 7 is left on the surface 3a of the mask 3, as shown in FIG. 26. In the event that the front end part 103 of the squeegee 101a further tightly presses the surface 3a of the mask 3 so as to prevent the solder paste 7 from remaining at the surface 3a of the mask 3, as shown in FIG. 27, the solder paste 7 is actually scraped, and the amount of the deformation of the front end part 103 of the squeegee is increased due to the larger contact pressure. Therefore, when the front end part 103 reaches the opening 4, a part of the front end part 103 enters the opening 4 subsequent to the restoration of the front end part 103, and undesirably scrapes the solder paste 7 already filled in the opening 4. Moreover, the powder solder included in the solder paste 7 facilitates the scraping. The amount of the solder paste 7 filled in the opening 4 is hence decreased and stable printing is prevented.
Under these circumstances, when printing is conventionally performed, a worker sets, adjusts and changes printing conditions based on experiments in order to fully achieve both the scraping operation and the filling operation so that the solder is printed paste stably. In other words, the printing conditions are set, adjusted and changed with a great deal of personal differences between workers, and maintenance of stable printing is difficult.
The present invention is devised to solve the above-described inconveniences, and an object of the present invention is to provide a solder paste printing apparatus and a solder paste printing method whereby a solder paste can be printed stably even when print time is increased as compared with the prior art.
In accomplishing this and other objects, according to a first aspect of the present invention, there is provided a solder paste printing apparatus wherein a squeegee device moves in a printing direction along a surface of a mask having openings formed therein, to thereby print and apply solder paste on the surface via the openings to a face of a circuit board positioned at a rear face of the mask, the squeegee device comprising: a filling squeegee having a front end kept in a noncontact state via a gap between the surface and the filling squeegee at the time of printing, and filling the solder paste into the openings while moving in the printing direction; and a scraping squeegee arranged behind the filling squeegee in the printing direction, moving in the same direction as the filling squeegee while keeping in touch with the surface at the time of printing thereby removing unnecessary solder paste on the surface.
According to a second aspect of the present invention, there is provided a solder paste printing apparatus according to the first aspect, wherein the scraping squeegee is provided for each direction behind the filling squeegee with respect to the printing direction in a case where the squeegee reciprocates relative to the surface of the mask.
According to a third aspect of the present invention, there is provided a solder paste printing apparatus according to the second aspect, wherein the filling squeegee is divided into two squeegees, one for each printing direction.
According to a fourth aspect of the present invention, there is provided a solder paste printing apparatus according to any one of the first to third aspects, further comprising an angle setting device for setting the scraping squeegee so that an angle of the squeegee in an axial direction thereof relative to the surface of the mask is optionally set to acute or obtuse angle.
According to a fifth aspect of the present invention, there is provided a solder paste printing apparatus according to the fourth aspect, wherein the scraping squeegee is inclined at the obtuse angle by the angle setting device.
According to a sixth aspect of the present invention, there is provided a solder paste printing apparatus according to the fourth aspect, wherein the scraping squeegee is inclined with the acute angle by the angle setting device thereby filling the solder paste into the openings as well as removing the unnecessary solder paste.
According to a seventh aspect of the present invention, there is provided a solder paste printing apparatus according to any one of the first to sixth aspects, wherein the scraping squeegee is disposed so that an extending direction of a line of contact between the scraping squeegee and the surface of the mask intersects with an extending direction of a side edge part defining one of the openings.
According to an eighth aspect of the present invention, there is provided a solder paste printing apparatus according to any one of the first to seventh aspects, wherein a face of the filling squeegee opposite to the surface of the mask forms a filling pressure face which is inclined upward from the front end in the printing direction so as to press the solder paste onto the surface and fill the solder paste into the openings.
According to a ninth aspect of the present invention, there is provided a solder paste printing apparatus according to the eighth aspect, further comprising a filling adjustment device for adjusting filling of the solder paste into the openings by varying at least one of a size of the gap and an intersection angle between the filling pressure face and the surface.
According to a tenth aspect of the present invention, there is provided a solder paste printing apparatus according to the ninth aspect, further comprising a filling pressure detector for detecting a change of a filling pressure of the solder paste filled into the openings at the time of printing, and a control device for controlling the filling adjustment device based on the filling pressure detected by the filling pressure detector.
According to an eleventh aspect of the present invention, there is provided a solder paste printing apparatus according to the tenth aspect, wherein the filling pressure detector is a reaction force detector for detecting a reaction force which is a sum of the filling pressures of the solder paste acting on the whole filling pressure face at the time of printing.
According to a twelfth aspect of the present invention, there is provided a solder paste printing apparatus according to the tenth aspect, wherein the filling pressure detector is a pressure detector set at the filling pressure face for detecting the filling pressure of the solder paste directly.
According to a thirteenth aspect of the present invention, there is provided a solder paste printing method comprising: moving a filling squeegee in a printing direction during printing while keeping a front end of the filling squeegee in a noncontact state via a gap between the filling squeegee and a surface of a mask having openings formed therein, thereby filling a solder paste on the surface to the openings; and removing unnecessary solder paste on the surface by a scraping squeegee moving in touch with the surface in the printing direction.
According to a fourteenth aspect of the present invention, there is provided a solder paste printing method according to the thirteenth aspect, wherein the scraping squeegee is set so that an angle of the scraping squeegee in an axial direction thereof relative to the surface of the mask is optionally an acute or obtuse angle.
According to the fifteenth aspect of the present invention, there is provided a solder paste printing method according to the fourteenth aspect, wherein, when the scraping squeegee is set to the obtuse angle, the scraping squeegee removes the unnecessary solder paste without adversely influencing the solder paste filled in the openings.
According to a sixteenth aspect of the present invention, there is provided a solder paste printing method according to the fourteenth aspect, wherein, when the scraping squeegee is set to the acute angle, the scraping squeegee fills the solder paste to the openings as well as removes the unnecessary solder paste.
According to a seventeenth aspect of the present invention, there is provided a solder paste printing method according to any one of the thirteenth to sixteenth aspects, wherein the scraping squeegee is set so that an extending direction of a line of contact between the scraping squeegee and the surface of the mask intersects with an extending direction of a side edge part defining one of the openings.
According to an eighteenth aspect of the present invention, there is provided a solder paste printing method according to any one of the thirteenth to seventeenth aspects, wherein a face of the filling squeegee opposite to the surface of the mask forms a filling pressure face. which is inclined upward from the front end in the printing direction so as to press the solder paste to the surface and fill the solder paste into the openings.
According to a nineteenth aspect of the present invention, there is provided a solder paste printing method according to the tenth aspect, wherein a change of a filling pressure of the solder paste when the solder paste is filled by the filling squeegee into the openings at the time of printing is detected by a filling pressure detector, and at least one of an intersection angle between the filling pressure face and the surface, and a size of the gap is adjusted based on the detected filling pressure, to thereby change printing conditions.